How contact tracing retains cryptographic privacy

Apple and Google are working on a new contact tracing protocol, built on top of Bluetooth broadcasts, that governments can use to build contact tracing mobile apps for end-users. It just released this week. We’ve heard lots of talk about how this is “privacy preserving”. You may think that it would be pretty challenging to keep a record of everyone you’ve seen, and compare that to a record of who everyone else has seen, without sharing any records of where anyone has been. This post covers how we can do it.